Unaudited Pro Forma Financial Information (re: 2019 and 2018 acquisitions) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Net Income, As Reported	$ 21,650	$ 20,509	$ 28,851	$ 25,710	$ 8,509	$ 29,242	$ 29,180	$ 26,208	$ 26,573	$ 24,184	$ 23,434	$ 26,427	$ 96,720	$ 93,139	$ 100,618
Basic EPS, As Reported (in dollars per share)	$ 0.71	$ 0.67	$ 0.95	$ 0.84	$ 0.27	$ 0.93	$ 0.93	$ 0.83	$ 0.84	$ 0.77	$ 0.74	$ 0.83	$ 3.17	$ 2.97	$ 3.19
Diluted EPS, As Reported (in dollars per share)	$ 0.71	$ 0.67	$ 0.94	$ 0.84	$ 0.27	$ 0.92	$ 0.92	$ 0.83	$ 0.84	$ 0.76	$ 0.74	$ 0.83	$ 3.16	$ 2.95	$ 3.17
Increase (decrease) in reported revenue													$ 82,800
Increase (decrease) in reported revenue, percentage													16.60%
Operating revenue	$ 146,183	$ 147,233	$ 144,275	$ 142,924	$ 136,327	$ 128,643	$ 124,626	$ 108,230	$ 104,681	$ 92,800	$ 87,387	$ 79,103	$ 580,615	$ 497,826	$ 363,971
Effect of Exchange Rate on Revenue													(9,300)	(6,900)
Parent, Curepet, Healthcare Magic, Vertex, Oakstone, I3, Qatarlyst
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Revenue, Pro forma													581,134	576,950
Net Income, Pro Forma													$ 96,678	$ 96,230
Basic EPS, Pro Forma (in dollars per share)													$ 3.17	$ 3.07
Diluted EPS, Pro Forma (in dollars per share)													$ 3.16	$ 3.05
Increase (decrease) in unaudited pro forma revenue													$ 4,200
Increase (decrease) in unaudited pro forma revenue, percentage													0.70%